During the last year, we have further characterized Zscan4 in mouse 2-cell embryos and in the subpopulation of mouse ES cells. We found that other 2-cell-specific genes are also highly expressed in Zscan4(+) cells. This indicates that the subpopulation of ES cells marked by Zscan4 expression shows some resemblance to 2-cell embryos, whereas the majority of ES cells show similarity to the Inner Cell Mass (ICM) cells in blastocysts. We are currently carrying out the Zscan4 knockdown and knockout experiments.